The present invention relates to a positioning unit for a radiation tomography apparatus in which a patient is so positioned that the part of body to be photographed may lie at a desired position.
Conventionally, a computerized tomography apparatus (to be referred to as a CT apparatus hereinafter) is known as one of the radiation tomography devices. The CT apparatus comprises an X-ray source 12 which radiates a flat, sector-shaped X-ray beam (fan beam X-ray) FX in a pulse-generating manner, a table 14 on which a patient P is laid, and a radiation detector 16 which opposes the X-ray source 12 with the table top 14 interposed. The radiation detector 16 is constituted by a plurality of radiation detecting elements D1, D2, . . . , Dm, . . . , Dn which are aligned parallel to each other, as shown in FIG. 1. In use of the unit of the tomography apparatus, a patient P is laid on the table 14, and the X-ray source 12 and the radiation detector 16 rotate periodically in the same circumferential direction through a predetermined angle about a part of body to be photographed of the patient P. With each rotation, X-rays radiated from the X-ray source 12 and transmitted through the patient P are detected by the radiation detector 16. Detection data for each exposure to the X-rays is collected and the obtained detection data is processed by a computer. The resultant data obtained by the computer processing are reconstructed to obtain a tomogram of the plane to be photographed. A zone 18 surrounded by the broken lines in the figure is a photographing region which is formed by the X-ray source 12 and the radiation detector 16.
In a CT apparatus, the part of body to be photographed of the patient P must overlap the photographing region (the region in which all the beams FX radiated from the X-ray source overlap, each time the X-ray source 12 and the radiation detector 16 are shifted by a predetermined angle). However, the photographing region cannot be visually observed. It is, therefore, difficult to match the plane to be photographed with the photographing region. In order to obtain a tomogram with higher precision, it is desired that a patient is so positioned that the part of body to be photographed lies at the center of the photographing region. However, this kind of positioning requires higher precision, resulting in more difficulty.
Conventional positioning units are classified into two types. A first kind of positioning unit adopts a system which uses a projector or phantom and in which the operator estimates the position of the patient with his eyes. A second kind of positioning unit adopts a system for accomplishing positioning by a microcomputer.
Among the conventional positioning units, units for positioning by a projector or a phantom require visual estimation for positioning the patient within the photographing region. Therefore, the positioning error may become great. Further, in devices of these types, the table top is manually moved upward or downward, requiring much labor. On the other hand, in the device for achieving positioning by a microcomputer, the operating cost is high and input data for the size of the patient is also required, resulting in time consuming operation.
None of the conventional positioning units satisfy all the requirements from the points of view of positioning precision, operability and operating cost.